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  which 
  forms 
  the 
  neural 
  or 
  spiaal 
  arch 
  of 
  the 
  embryo, 
  or 
  the 
  

   arch 
  in 
  which 
  the 
  spinal 
  cord 
  is 
  formed. 
  

  

  Within 
  the 
  exodcrm, 
  and 
  from 
  the 
  within-lying 
  cells 
  of 
  

   the 
  blastoderm, 
  two 
  other 
  layers 
  are 
  subsquently 
  formed 
  — 
  

   the 
  mesoderm 
  or 
  middle 
  layer, 
  and 
  the 
  endoderm 
  (hypoblast) 
  

   or 
  internal 
  layer. 
  From 
  the 
  folding 
  in 
  and 
  union 
  of 
  these 
  

   two 
  layers 
  in 
  conjunction 
  with 
  the 
  exoderm, 
  a 
  second 
  arch, 
  

   the 
  body 
  or 
  alimentary 
  cavity, 
  is 
  formed. 
  The 
  further 
  

   growth 
  of 
  the 
  embryo 
  then 
  takes 
  place 
  through 
  differentia- 
  

   tions, 
  either 
  slow 
  or 
  more 
  rapid, 
  in 
  the 
  three 
  layers 
  making 
  

   up 
  these 
  two 
  arches. 
  The 
  first 
  indication 
  in 
  the 
  egg 
  of 
  the 
  

   Smelt 
  of 
  anything 
  approaching 
  a 
  fish 
  or 
  animal 
  in 
  form, 
  is 
  

   represented 
  in 
  Fig. 
  1, 
  PI. 
  11. 
  

  

  Here 
  the 
  folding 
  in 
  of 
  the 
  exoderm 
  to 
  form 
  tlie 
  neural 
  

   arch 
  is 
  nearly 
  completed, 
  tlic 
  anterior 
  and 
  posterior 
  ends 
  

   being 
  still 
  open_, 
  althougli 
  only 
  the 
  anterior 
  end 
  can 
  be 
  seen. 
  

   At 
  this 
  anterior 
  end 
  is 
  seen 
  quite 
  an 
  enlargement, 
  which 
  

   represents 
  the 
  swelling 
  of 
  the 
  cerebral 
  vesicles, 
  or 
  what 
  will 
  

   eventualh' 
  become 
  lobes 
  of 
  the 
  brain. 
  This 
  end 
  increases 
  

   still 
  more 
  in 
  size, 
  the 
  lobes 
  appearing 
  to 
  swell 
  out 
  with 
  a 
  

   more 
  rounded 
  appearance, 
  and 
  just 
  before 
  the 
  anterior 
  end 
  

   of 
  the 
  neural, 
  canal 
  closes, 
  round, 
  disk-like 
  markings 
  appear 
  

   upon 
  the 
  middle 
  portion 
  of 
  the 
  sides 
  of 
  the 
  head— 
  as 
  we 
  shall 
  

   hereafter 
  call 
  this 
  end 
  of 
  the 
  embryo. 
  These 
  markings, 
  Fig. 
  

   2, 
  PI. 
  II, 
  represent 
  the 
  first 
  indications 
  of 
  the 
  eye-balls. 
  

  

  The 
  head 
  presents, 
  at 
  this 
  period, 
  a 
  very 
  dumpy 
  or 
  club- 
  

   shaped 
  appearance, 
  with 
  its 
  prominent 
  lobes 
  and 
  rather 
  thick 
  

   lip-like 
  edges 
  of 
  the 
  closing 
  neural 
  canal. 
  Very 
  nearly 
  witli 
  

   the 
  appearance 
  of 
  the 
  eye 
  protuberances, 
  or 
  very 
  soon 
  after, 
  

   can 
  be 
  seen 
  a 
  cylindrical 
  tube 
  containing 
  two 
  rows 
  of 
  large 
  

   cells 
  side 
  by 
  side, 
  Avhich 
  barely 
  toucli 
  in 
  any 
  part, 
  and 
  are 
  

   somewhat 
  scattered 
  posteriorly, 
  extending 
  nearly 
  the 
  entire 
  

   length 
  of 
  the 
  embryo, 
  just 
  along 
  the 
  ventral 
  side 
  of 
  the 
  neu- 
  

   ral 
  arch. 
  This 
  is 
  the 
  notochord 
  or 
  the 
  axial 
  portion 
  of 
  the 
  

   future 
  back-bone. 
  Fig, 
  2, 
  PL 
  III. 
  It 
  ends 
  anteriorly, 
  with 
  

   a 
  quite 
  abruptly 
  bent 
  extremity, 
  just 
  on 
  a 
  line 
  with 
  the 
  pos- 
  

  

  